WO2006128850A1

WO2006128850A1 - Device for detecting a capacitance variation and capacitive sensor for detecting the presence of a user using such a device

Info

Publication number: WO2006128850A1
Application number: PCT/EP2006/062693
Authority: WO
Inventors: Xavier Hourne
Original assignee: Valeo Securite Habitacle
Priority date: 2005-06-01
Filing date: 2006-05-30
Publication date: 2006-12-07
Also published as: US8570054B2; EP1896801A1; FR2886725B1; WO2006128850A9; JP5094714B2; US20100231240A1; FR2886725A1; JP2008542734A

Abstract

The invention concerns a device for detecting a capacitance variation of a variable capacitive structure (Cx), comprising: means for generating voltage pulses, means for charging the variable capacitive structure (Cx) from said voltage pulses, means for discharging said capacitive structure (Cx) to a reference capacity (Cs), means for detecting a voltage threshold at the terminals of the reference capacity (Cs), means for determining a number of charges and/or discharges of said variable capacitive structure (Cx) corresponding to said voltage threshold, and means for detecting a variation of said number of charges and/or discharges relative to a number of charges and/or discharges previously obtained. The invention is applicable in any technical field involving user detection, for example in a vehicle door handle inside a vehicle or a housing.

Description

"Device for detecting a capacitance variation and capacitive sensor for detecting the presence of a user, implementing such a device"

The present invention relates to the realization of a device for detecting a capacitance variation of a variable capacitive structure. It also aims a capacitive sensor of presence detection of a user, implementing such a device.

The field of the invention relates to the detection of the presence of a user, especially in the automotive sector. In fact, user detection devices are becoming more and more an integral part of embedded security and control systems and the detection of a user is considered as the awakening of a complete system that can be put on standby in the absence of a user. This standby of a complete system avoids soliciting this system during the time when no user is present, which contributes to a significant saving of energy.

There are currently many presence detection systems of a user. There may be mentioned systems based on optical sensors, temperature sensors, electrodes and capacitive sensors. Systems based on capacitive sensors are based on an assessment of the capacitance variation of a capacitive variable structure depending on the presence or absence of one or more users.

Currently there are capacitive charge transfer sensors like the sensors described in the book "CAPACITIVE SENSORS" by Larry K. Baxter, published by The Institute of Electrical and Electronics Engineers, Inc. The sensors presented in this book are used to measure the variation of the capacitance of a variable capacitance C1, thanks to a capacity C2 referred to as reference. By performing a cycle consisting of a large number of charges and discharges of the capacitor C1 in the reference capacitor C2 until a fixed voltage threshold is reached across the reference capacitor C2, it is possible to detect the variation of the capacitance. capacitance of Cl compared to the previous cycle by estimating the variation in the number of discharges of the capacitance Cl in the capacity C2 that had to be made to reach the voltage threshold across C2.

These sensors involve a switch component wc, which directs the current to perform first the load of the capacitor C1 and then the discharge of the capacitor C1 in the capacitor C2 and vice versa. But this switch component is an element that is difficult to achieve and whose realization is very expensive.

There is therefore a technical problem of producing a current switching circuit so that it is possible to switch from a load circuit of a variable capacitive structure whose capacitance variation to a circuit is to be detected. discharge of this capacitive structure.

In the following, a variable capacitive structure is understood to mean any hardware configuration constituted by a capacitive electrode integrated in an object or in a system and by its close environment directly or indirectly connected to a mass reference, this environment possibly including a part of the body. a user whose presence must be detected.

The invention proposes to remedy the aforementioned problems by a device for detecting a variation of the capacitance of a variable capacitive structure, comprising: means for generating voltage pulses, means for charging the variable capacitive structure from said voltage pulses, means for discharging said variable capacitive structure to a reference capacitor, means for detecting a voltage threshold across the reference capacitor, means for determining a number of charges and / or discharges of said structure capacitive variable corresponding to said voltage threshold, and means for detecting a variation of said number of charges and / or discharges with respect to a number of charges and / or discharges previously obtained. The detection device according to the invention constitutes an innovative approach in the detection of the variation of the capacitance of a variable capacitive structure, since the electric circuit implemented to ensure the charging and discharging of the variable capacitive structure is very simple and can be achieved with inexpensive elements.

There is no physical switching between the charging and discharging circuit of the capacitor, which advantageously makes it possible to dispense with an element such as a switch to perform such an operation. Indeed, this circuit is arranged to ensure a unidirectional transfer of energy from the variable capacitive structure to the reference capacitor, which makes it possible to separate the load from the variable capacitive structure Cx, whose capacitance variation is to be measured, from its discharge and thus ensure a total discharge of the variable capacitive structure to the reference capacitor Cs. Thus, it is avoided that a portion of the energy contained in the variable capacitive structure returns to the pulse generating means, and is not taken into account in the detection of the capacitance variation of the variable capacitive structure Cx.

Such energy transfer is advantageously carried out by a component that only allows a unidirectional transfer of the current. Such a component may be a diode D1 or any other element that can fulfill such a role. Thus, being placed in series between the pulse generating means and the other means forming part of the device according to the invention, the diode D1 passes the current of the pulse generating means to the variable capacitive structure Cx during charging. of the variable capacitive structure Cx. Upon discharge of the variable capacitive structure Cx, it prevents the current from returning to the pulse generating means or any means other than the reference capacitor Cs. Thus, the diode D1 makes it possible to channel the discharge of the variable capacitive structure Cx towards the reference capacitor Cs. Knowing that the operation of switching from the charging circuit to the discharge circuit and vice versa is an operation performed a very large number of times during the detection of the variation of the capacitance, the fact of replacing such an operation by an element such as a diode D1 advantageously simplifies the control of such a circuit.

The voltage pulses necessary for charging the variable capacitive structure Cx whose capacitance variation is to be measured can advantageously be achieved by a clock whose pulse duration is sufficiently large to allow the variable capacitive structure to fully charge. In addition, the duration of the pulses must not be too great to load the reference capacitor Cs, which could then eventually distort the detection of the variation. In this way, the realization of the pulses is advantageously performed by simple and small clock circuit that can be easily integrated into a system.

For the realization of voltage pulses of duration adapted to the total load of the capacity, it is also possible to use a clock circuit delivering a square signal, followed by a high pass filter. In this way the square signal at the output of the clock is injected into the high pass filter, and converted into pulses by the high pass filter. It will be necessary to ensure that the clock and the filter passes high have characteristics (period, amplitude, etc ..) adapted to generate voltage pulses long enough to load the variable capacitive structure Cx and short enough not to load the reference capacitor Cs, which is mainly to be loaded by the discharges of the variable capacitive structure Cx.

The discharge means of the variable capacitive structure Cx may advantageously comprise a resistor Rs. This resistor Rs, placed in parallel with the variable capacitive structure Cx, downstream of the diode D1 and upstream of the reference capacitor Cs, enables have control over the discharge of the variable capacitive structure Cx. Thus, this resistance makes it possible to control the time constant T1 = Rs * Cx, which must be greater than the duration of the pulses generated by the pulse generating means.

The device according to the invention also comprises means for detecting a voltage threshold across the reference capacitor Cs. Thus, after multiple discharges of the variable capacitive structure Cx, whose capacitance variation is to be measured, in the reference capacitor Cs, the voltage across the capacitance increases and reaches a voltage threshold. The device according to the invention further comprises means for determining the number N of discharges which made it possible to reach the fixed voltage threshold across the reference capacitor Cs. These means comprise a counter for counting the number of charges and / or discharges and / or the number of pulses generated by the pulse generating means.

It is useful to specify that the reference capacity Cs is such that the variable capacitive structure Cx can be discharged a large number of times in Cs. This supposes that the reference capacity Cs has a capacitance greater than Cx.

Thus one can count the number N (t) of times that the variable capacitive structure Cx has discharged in the reference capacitor Cs, so that the voltage across the reference capacitor Cs reaches a fixed voltage threshold. The device according to the invention advantageously comprises means for detecting a variation of number N (t) of charges and / or discharges of the variable capacitive structure Cx corresponding to the fixed voltage threshold across the reference capacitor Cs with respect to a number N (tl) previously obtained. The variation in the number of discharges, of the variable capacitive structure Cx in the reference capacitor Cs, corresponding to a voltage threshold across the reference capacitor Cs, is directly related to the variation of the capacitance of Cx. Thus one can detect the variation of the capacitance of Cx. Once the detection operations are completed the reference capacity can be discharged through a switch SWl, to allow a new phase of detection.

An exemplary application, in no way limiting, of the device according to the invention may be the detection of the variation of the capacitance between a detection electrode disposed in an equipment and the environment of the electrode. Thus, the variable capacitive structure Cx will then be the capacitance constituted, on the one hand, by the detection electrode and on the other hand by its environment. The environment of the sensing electrode can then be a wall, a human presence, a car, or any other real-life object. This electrode can be incorporated into a car door handle to serve as a user presence detection device. The detection device can be advantageously integrated into the handle. Other advantages and characteristics will appear on examining the detailed description of a non-limiting embodiment, and the appended drawings in which:

FIG. 1 illustrates an exemplary mode of operation of a presence detection system implementing the device according to the invention; FIG. 2 represents an exemplary embodiment of a detection device according to the invention; and

FIG. 3 shows timing diagrams of signals characteristic of the detection device according to the invention.

A non-limiting mode of implementation of a capacitance variation detection device according to the invention relates to a presence detection system disposed in a car door handle.

An electrode is placed in the handle of the door of a car. This electrode and its environment represent the variable capacitive structure Cx whose capacitance variation is to be determined. The device according to the invention, also arranged in the handle of the door, makes it possible to measure the variation of the capacitance between the electrode and its environment. The environment of the electrode is composed of any object or being alive near the car and the car itself.

The capacitance variation of the capacitive structure Cx between the electrode and its environment is measured at each instant according to the operating mode shown in FIG. 1. As soon as a rapid variation of the capacitance is detected, an activation of the output signals of the presence detection system is generated.

The variation of the capacitance is measured by the device according to the invention through a quantity which is the number N of clock periods necessary to reach a voltage threshold across a reference capacitor Cs. However, there may be a slow variation in capacitance, due to temperature and humidity conditions and any other factors unrelated to the presence of a potential user. It is therefore important to differentiate these variations from those caused by a potential user. This is why the variation of the capacitance is considered with reference to the value N measured at the previous instant. In this way, the slow variations of capacitance Cx due to the environment and the electrode are taken into account.

FIG. 2 shows a particular embodiment of an electronic charging and discharging circuit of the variable capacitive structure Cx, implemented in a detection device according to the invention, while FIG. 3 represents a time evolution of different signals observed in this circuit: the clock signal, the voltage pulses, the voltage across the variable capacitive structure Cx and the voltage across the reference capacitor Cs. The reference capacitor Cs is loaded as the variable capacitive structure Cx charges and discharges with the pulses generated by a clock associated with a high pass filter. This circuit makes it possible to charge the reference capacitor Cs with discharges of the variable capacitive structure Cx. Once the fixed voltage threshold across Cs is reached, the number N of clock period is recorded which made it possible to reach this threshold voltage. The number N is compared to its previously obtained value. If there is a slow variation of the value of N, it means that no user is present near the car so the operation is repeated. The value of N is recorded for comparison with the value of N which will be obtained in the next step. If there is a rapid change in the value of N, then this means that a potential user is present near the handle of the car. An output signal is then emitted to trigger the following operations.

Such a system for detecting the presence of a user using the device according to the invention advantageously makes it possible to avoid the wall effect during the opening of the door. Indeed, the capacitance variation detection system can detect very rapidly variations in the order of 1%, can anticipate the approach of a user and therefore the opening of the door before the handle is actuated.

The detection device according to the invention is not limited to the embodiment and to the application which have just been described, and can be implemented in other applications. In particular, it could replace actuating buttons, for example, the ignition button of a ceiling light in a car, which would turn on the light without having to look away to find the exact location of the button ignition.

Claims

1) Apparatus for detecting a variation of the capacitance of a variable capacitive structure Cx, comprising: - means for generating voltage pulses, means for charging the variable capacitive structure (Cx) from said voltage pulses, means for discharging said variable capacitive structure

(Cx) towards a reference capacitor (Cs), - means for detecting a voltage threshold across the reference capacitor (Cs), means for determining a number of charges and / or discharges of said variable capacitive structure (Cx) corresponding to said voltage threshold, and - means for detecting a variation of said number of charges and / or discharges with respect to a number of charges and / or discharges previously obtained.

2) Device according to claim 1, characterized in that it further comprises means for a unidirectional transfer of energy of the variable capacitive structure (Cx) to the reference capacitor (Cs).

3) Device according to claim 2, characterized in that the means for providing a unidirectional transfer of energy from the variable capacitive structure (Cx) to the reference capacitor (Cs) comprises a diode Dl.

4) Device according to any one of the preceding claims, characterized in that the pulse generating means comprise a clock.

5) Device according to any one of the preceding claims, characterized in that the pulse generating means comprise a high pass filter. 6) Device according to any one of the preceding claims, characterized in that the discharge means of the variable capacitive structure (Cx) comprise a resistor (Rs * Cs).

7) Device according to claim 6, characterized in that the resistor (Rs) has a value such that the time constant (Rs * Cx) is greater than the duration of the voltage pulses and less than the half period of the pulses.

8) Device according to any one of the preceding claims, characterized in that the means for detecting the number of charges and / or discharges of the variable capacitive structure (Cx) comprise a counter.

9) Device according to one of the preceding claims, characterized in that the reference capacitor (Cs) has a capacitance greater than that of the variable capacitive structure (Cx).

10) Device according to any one of the preceding claims, characterized in that it further comprises switch means (SWl), controlled to discharge the reference capacitor (Cs), once the voltage at its terminals has reached a predetermined voltage threshold.

11) Capacitive sensor for detecting the presence of a user of a device, implementing a detection device according to any one of the preceding claims, characterized in that the capacitive structure (Cx) whose capacitance variation is detected comprises a detection electrode disposed within said equipment, the capacity of the capacitive structure being determined between said detection electrode and the environment close to said detection electrode.

12) Capacitive sensor according to claim 11, characterized in that the equipment in which the detection electrode is disposed is a door handle of a vehicle. 13) Capacitive sensor according to claim 12, characterized in that the detection device according to any one of claims 1 to 10 is integrated in the handle.